It is well known that salt bath liquid, in which steel such as stainless steel is immersed for the purpose of descaling, contains nitric acid radicals and toxic chromates in high concentrations. In order to isolate the metallic salts, it is most common to make the liquid strongly acidic, and then add a strong reducing agent so as to allow the acidic liquid to resume alkalinity. However, this prior art method has drawbacks which limit its utility. Firstly, the reducing agent involved is relatively expensive and yet it is not recovered. Secondly, the cumbersome and time-consuming character of the above-described procedures results in high costs.
A great convenience will be afforded to the isolation of salt radicals contained in the waste liquid, if these salt radicals are converted into insoluble matters as a result of reacting on a compound added to the waste liquid. However, it is difficult to come upon such a compound, and the technique which can be put to practical use for the isolation of salt radicals from the waste liquid has not been proposed as yet.
When the waste liquid contains free alkalis of especially high concentration and soluble metallic salts of considerably high concentration, it is most common to adjust the pH-value of the waste liquid in order to convert metal ions into insoluble matters with a view to isolating them from the waste liquid. In order to adjust pH-value of waste liquid, it is known to add a large quantity of an acid not only to neutralize the alkalis but even to make the liquid acidic, and then to add a strong reducing agent so as to decrease the valencies of metals.
Japanese Patent Unexamined Publication No. 2-145786 describes a method of recovering free acids from used pickling liquor, wherein a semipermeable membrane is used for the purpose of recovery. The reducing power of ferrous ions (Fe.sup.++) remaining in the used pickling liquor is utilized.
Apart from this prior art method, it is always desirable to provide a method of effectively removing metallic salts coexisting with a large quantity of nitric acid radicals in waste liquid discharged from a salt bath furnace.
When steel is immersed in salt bath liquid for the purpose of surface treatment, a reaction product collects on the surface of the steel. After taking out the steel from the salt bath furnace, the reaction product has to be removed with a large quantity of water.
In current practice, the salt bath liquid for the surface treatment of steel consists of sodium hydroxide and sodium nitrate, with the equivalent ratio of sodium hydroxide to sodium nitrate substantially falling within the range between 6:4 and 7:3. This means that sodium hydroxide prevails over sodium nitrate. Furthermore, the gram equivalent of sodium hydroxide tends to be larger than the total amount of acid radicals used in the surface treatment process. For these reasons, industrial sewage tends to be a considerably strong base, which has to be neutralized by an expensive acid.
The trouble is that not only an expensive agent but also chromic ions and manganous ions dissociated from steel are contained in high concentrations in waste water discharged from the site of the above-mentioned rinsing process. In order to remove these components, an expensive acid has to be added in the first place so as not only to neutralize alkaline components existing in large quantities but even to make the waste water acidic, and then a strong reducing agent has to be added to the acidic waste water so as to decrease the valencies of metals, allow the acidic waste water to resume alkalinity, and convert the metal ions into insoluble matters so that they may be isolated from the waste water.
The cumbersome and time-consuming character of the above-described procedures not only results in high costs but also poses the following problems:
A) A large quantity of expensive sodium hydroxide is neutralized simply to produce a salt of little value and usefulness. No one has been thoughtful enough to think of a method of effectively recovering the expensive sodium hydroxide. PA1 B) In addition to sodium hydroxide, industrial sewage contains nitrates, which in turn contain a large quantity of nitric acid radicals. Consequently, the quantity of industrial sewage which can be discharged into sewer pipes is subject to regulation, although there are regional differences in the severity of regulation. It is keenly desirable, therefore, to provide a method by which nitric acid radicals can be effectively removed before the industrial sewage is discharged into sewer pipes. Since nitric acid radicals are a relatively expensive agent, they are worth being isolated for the purpose of recycling them as nitric acid. If the above-described acid bath liquid is subjected to electrolysis, not only metals but also sodium, which has reacted on nitric acid radicals, will be removed. PA1 C) The above-described acid bath liquid further contains a large quantity of toxic chromates and manganates, which have to be removed for the prevention of environmental pollution. In order to remove these metallic salts, they have to be converted into insoluble matters. For this purpose, the largest expenses quota has to be given to the purchase of an acid to be used for neutralizing the alkalis coexisting with the metallic salts. Thus the problem mentioned in this paragraph and the problem mentioned in paragraph A) are inseparably related to each other. PA1 a) To provide a method of efficiently isolating free sodium hydroxide from the above-described rinsing water so that the isolated sodium hydroxide may be recycled and the necessity of purchasing an acid to be added for the purpose of neutralization may be obviated. PA1 b) To provide a method by which Na.sup.+ remaining as the cations of sodium nitrate in the rinsing water after the isolation of free sodium hydroxide can be removed. PA1 c) To provide a method by which metal ions remaining in the rinsing water after the isolation of free sodium hydroxide can be reduced so as to be easily removed. PA1 (A) The chief ingredients of the salt bath liquid are sodium hydroxide and sodium nitrate. Metal ions dissociated from steel are contained in high concentrations. PA1 (B) Since the salt bath liquid is held at 400.degree. C. to 600.degree. C., chromium compounds dissociated from steel are dissolved in a stable hexatomic state. PA1 (C) Other metallic salts also exist. These compounds are carried by quenching water to one end of the rinsing tank where waste water is discharged. Except hexatomic chromium, they are insoluble matters. In brief, the salt bath liquid is composed of: PA1 (D) On many occasions, a nitric acid bath is attached to a salt bath in a plant where stainless steel is subjected to surface treatment. Nitric acid bath liquid must be discarded when metals are dissociated from stainless steel to such an extent that the nitric acid bath is made functionally useless thereby. However, this bath liquid contains ferrous ions (Fe.sup.++) having a reducing power strong enough to reduce hexatomic chromium to an innoxious hydroxide.
When, in an acidic atmosphere, ferrous ions (Fe.sup.++) coexist with metal ions which remain in a dissolved state even in an alkaline atmosphere, a simplified process of reducing such metal ions is available. Since each Fe.sup.++ loses one electron when it is oxidized to Fe.sup.+++, all that has to be done is to allow the metal ions to gain electrons so as to allow these metal ions to decrease valencies.
Whether or not this simplified process comes off well depends upon the conditions of pickling liquor in a pickling bath incorporated in the same production line as the salt bath furnace involved. The conditions to be fulfilled by the pickling liquor are that it should contain 0.7 to 1.0N free acid radicals so as to be strongly acidic, that it should be kept at 40.degree. to 60.degree. C. so as to allow the reaction to smoothly proceed, and that it should contain a large quantity of Fe.sup.++ required as a reducing agent. For the effective pickling of stainless steel, a pickling agent should preferably contain both Fe.sup.++ and Fe.sup.+++ in a suitable ratio. If the above-described conditions of pickling liquor are fulfilled, they will obviate the necessity of purchasing an expensive reducing agent and having trouble with where to dump the sludge resulting from the reducing process.
On many occasions, the process of reducing Cr.sup.6+ to Cr.sup.+ by Fe.sup.++ is allowed to proceed in a bath liquid in which stainless steel is descaled. It has been found that on such occasions a change in the characteristics of metal surfaces is caused by reduced chromium, which sticks to the surfaces of activated metal at the time of descaling.
Salt radicals, of which the salt bath liquid is composed, cannot be converted into insoluble matters even if they are neutralized, in spite of the fact that these salt radicals have to be removed from the waste liquid in order to prevent the eutrophication of lakes.
Sodium nitrate, which is the chief ingredient of the salt bath liquid, is soluble. Enrichment is the only method of removing sodium nitrate from the waste liquid. Enriched sodium nitrate cannot be recycled until it is converted into an anhydrous salt.
Furthermore, the high-temperature salt bath converts chromium molecules, which are one of the ingredients of stainless steel, into Cr.sup.3+ and further into water-soluble and toxic Cr.sup.6+, which gives rise to a problem in connection with the disposal of industrial waste matter.
In order to remove Cr.sup.6+, it is most common to reduce Cr.sup.6+ to Cr.sup.3+ by a reducing agent and allow the ions to cohere. However, the technique which can be put to practical use for the removal of nitric acid radicals from the waste liquid has not been proposed as yet.
It has also been proposed to use a ferrous salt as a reducing agent for reducing Cr.sup.6+ to Cr.sup.3+.
Japanese Patent Application No. 63-9880 describes a method of recovering free acids from a used pickling liquor, wherein a semipermeable membrane is used for the purpose of recovery. Waste liquid, the chief ingredient of which is metallic salts, is utilized as a reducing agent. However, this Application does not show a method of enriching the nitric acid radicals contained in the quenched waste liquid and recovering free nitric acid and sodium hydroxide so that they can be recycled.
Chemicals contained in salt bath liquid are relatively expensive. Furthermore, it is difficult to isolate them from the waste liquid.
In current practice, acid bath liquid containing nitric acid as a chief ingredient is simply neutralized and discarded. However, a portion of iron contained in this kind of acid bath liquid is an effective reducing agent which acts on Cr.sup.6+. An effective utilization of this portion of iron will result in a curtailment of the expenses quota to be given to the purchase of a chemical agent for reducing Cr.sup.6+.